Recently, a high corrosion resistance is required for a steel plate used for a car body, and there is a growing tendency that a surface-treated steel plate having a high corrosion resistance is used instead of a cold-rolled steel plate heretofore used.
As the surface-treated steel plate, a zinc-deposited steel plate can be first of all mentioned. In this deposited steel plate, in order to increase the corrosion resistance, the amount deposited of zinc should be increased. However, increase of the amount deposited of zinc results in degradation of the workability and weldability. As a steel plate overcoming this defect, there has been studied and developed a zinc alloy-deposited steel plate or multiple layer-deposited steel in which at least one element selected from the group consisting of Ni, Fe, Mn, Mo, Co, Al and Cr is added, and in steel plates of this type, the corrosion resistance can be improved over that of the above-mentioned zinc-deposited steel plate without degradation of the workability or weldability. When a steel plate is applied to a bag structure portion or curved portion (hemmed portion) of an inner plate of a car, a high corrosion resistance is required for the surface of the steel plate, and the above-mentioned zinc alloy-deposited steel plate or multiple layer-deposited steel plate is still insufficient in the corrosion resistance. A rust-preventive coated steel plate having a zinc-rich coating, as disclosed in Japanese Patent Publication No. 24230/70 or No. 6882/72, has been studied and developed as a steel plate having a high corrosion resistance, and a steel plate called "Zincrometal" is known as a typical instance of the steel plate of this type. However, even in this rust-preventive coated steel plate, peeling of the coating is often caused in a processed portion, for example, a portion subjected to press-forming, resulting in degradation of the corrosion resistance. Therefore, this coated steel plate is not sufficiently satisfactory as a highly anti-corrosive, rust-preventive coated steel plate meeting the requirement for a car body material.
Under this background, we newly developed a steel plate having a protecting film free of a metal powder such as a Zn powder in the form of a thin film (having a thickness smaller than several microns) based on the consideration that the improvement of the capacity of the rust-preventive coated steel plate by the zinc-rich coating layer is limited, and we proposed this new steel plate in Japanese Patent Application Laid-Open Specifications No. 224174/83, No. 50179/85, No. 50180/85 and No. 50181/85. This steel plate is obtained by forming a chromate film and an organic composite silicate film as the topmost layer on a zinc-deposited or zinc alloy-deposited steel plate as the base. This coated steel plate is especially excellent in the workability and corrosion resistance.
A part (trunk lid, hood or the like) of the inner face of a car body is sometimes coated with at least two coating layers including a topcoat formed on a cation electrodeposition coating. In case of the above-mentioned coated steel plate previously proposed by us, it is apprehended that the adhesion is insufficient in case of this multi-layer coating. Therefore, in Japanese Patent Application Laid-Open Specification No. 174879/85, we proposed a proces in which the above-mentioned steel plate is improved and a rust-preventive steel late for multi-layer coating excellent in the adhesion of the multi-layer coating is prepared.
According to this process, an organic polymer film is sufficiently crosslinked by baking at a high temperature of 250.degree. to 350.degree. C. to ensure an excellent adhesion to a multi-layer coating. If crosslinking of the polymeric film is insufficient, the film is softened and swollen by an alkali generated in the interface at the cation electrodeposition, resulting in degradation of the adhesion of the coating. This defect is overcome by crosslinking by high-temperature baking.
From the results of research made by us afterward, it was found that although in the above-mentioned steel plate an excellent adhesion to a multi-layered coating having at least two coating layers can be ensured by baking at a high temperature exceeding 250.degree. C., the so-called bare corrosion resistance (uncoated corrosion resistance), that is, the corrosion resistance on the supposition that an electrodeposition coating layer is hardly formed, is insufficient and if the surface treatment coating film is damaged, for example, if a cross cut extending to the iron substrate is formed or the steel plate is subjected to deep-draw forming or draw beading, the bare corrosion resistance is somewhat inferior to that of the steel plate as disclosed in Japanese Patent Application Laid-Open Specification No. 224174/83 mentioned above.
In addition to excellent workability and weldability, the following properties are required for a highly corrosion-resistant surface-treated steel plate for a car body:
(1) A high corrosion resistance of a portion where an electrodeposition coating is hardly formed, such as a bag structure portion or a hemmed portion, that is, a high bare corrosion resistance. PA1 (2) Good coating propoerties (high adhesion and high corrosion resistance of the coating) of a multi-layer coating including at least two coating layers (cation electrodeposition coating layer and topcoat layer) on the inner face of a trunk lid or hood.
At the present, the requirement for improving the corrosion resistance in a car body is increasing, and the above-mentioned steel plate cannot be regarded as satisfying this requirement sufficiently.
The present invention is to solve the foregoing problem and provide a highly corrosion-resistant surface-treated steel plate which has good workability and weldability and is excellent in bare corrosion resistance, adhesion to a multi-layer coating and corrosion resistance of the coating.